1. Field of the Invention
This invention is directed to a method for performance monitoring and equalization of wavelength division multiplexed (WDM) optical networks with optical add/drop multiplexers (OADM).
2. Background Art
Performance monitoring and equalization is one of the key issues in the development of optical dense DWDM (wavelength division multiplexed) networks. In the last decade, transmission rates of data signals have increased progressively, which demands faster and more complex transmission systems.
To increase the capacity of transmission using the existing fiber plant, many optical network providers adopted WDM/DWDM technology. On the other hand, for high transmission rates, i.e. rates over 40 or 80 Gb/s, system performance degradation caused by noise and optical path distortions are usually difficult to measure, making the performance evaluation complicated.
In optically amplified WDM/DWDM system, signals travelling on different channels have a different gain and a different signal-to-noise ratio due to the unflat gain profile of the optical amplifiers provided along the path of the signal, different parameters of the transmitter-receiver pairs, optical distortion, etc.
The bit error rate (BER) and the optical signal-to-noise ratio (OSNR) have usually been used as parameters for the evaluation of the characteristics of an optical fiber communication system.
BER is defined as the ratio between the number of the erroneously received bits to the total number of bits received over a period of time. The BER includes information on all impairments suffered by the signal between the transmitter and receiver, i.e. both noise and distortion information. OSNR represents the noise characteristics of an optical system and is the ratio between the signal and noise levels.
Individual channels generally experience different OSNR and BER, even if the input power is equal at the transmitter site. This is because the transmitter-receiver pairs do not have identical characteristics for all channels, the optical amplifiers have unflat gain and noise profiles, the passive components connected over the transmission link have wavelength-dependent characteristics. etc. Moreover, the active and passive network elements have different characteristics, further contributing to the differences between the performance of the respective transmission channels.
It is important to detect accurately the performance of individual optical channels for many reasons, including improved control of optical amplifiers, signal tracking at the optical layer, monitoring accumulation of optical noise in a link with cascaded amplifiers. and most importantly, equalization of the channelperformance. Without equalization, some optical channels may have much more tolerance to noise and loss than others, which may fail to meet the provisioned performance parameters.
Equalization based on OSNR is a straight-forward method. OSNR may be measured using an optical spectrum analyser or other methods. The drawback associated with this method is that it does not consider the effects of the optical distortion and electrical noise.
Another method used currently for equalization is based on measuring the BER margin for each channel. While equalization based on BER margin is generally more sophisticated, it is more accurate than OSNR method. BER margin can be measured using, for example, xe2x80x9cnoise loadingxe2x80x9d method disclosed in U.S. patent application Ser. No. 08/934,969 (Khaleghi, filed Sep. 19, 1997 and assigned to Northern Telecom Limited).
The article xe2x80x9cEqualization in amplified WDM lightwave transmission systemsxe2x80x9d, A. R. Chraplyvy et al., IEEE Photonics Technology Letters, Vol. 4, Nov.8, 1992, pp920-922 and U.S. Pat. No. 5,225,922 (issued on Jul. 6, 1993 to Chraplyvy et al., and assigned to ATandT Bell Laboratories) provides a method for adjusting the optical powers at the transmitter end of a WDM transmission link to obtain the same OSNR for all channels at the receiver end. The adjustment of a particular channel takes into account the total power of all channels and the end-to-end gain for that channel normalized by the end-to-end gain for all channels. A telemetry link is necessary for conveying the measurements between the two ends of the link.
Latest advances in the opto-electronics have been conducted towards the replacement of electrical add/drop multiplexers (ADM) with optical ADMs (OADMs), where an entire optical channel is dropped or added at the OADM site according to the wavelength. As OADMs are transparent to the signal rate, they can be used in DWDM networks with different SONET/SDH rates.
Performance monitoring and equalization of optical links with multiple OADMs becomes more challenging also because a wavelength can be reused in the same link. Furthermore, the OADMs are installed at locations specified by the customer.
It is known to use the xe2x80x9cglass-throughxe2x80x9d method in end-to-end systems provided with a small number of OADMs This method involves short-connecting the drop and add ports of the OADM with a fiber patch cord, so that they appear as a single channel from an equalization point of view. The equalization of the channels is then performed generally based on OSNR or BER margin. After equalization, the patchcord between the add and drop ports is removed and the affected channels are reconnected to their respective transmitter and receiver. Since, the xe2x80x9cglass throughxe2x80x9d method ignores entirely the existence of the OADM from the point of view of performance monitoring, after equalization. the add/drop channels will most probably have much more margin-to-failure than the other channels. Furthermore, this method cannot be used for systems that have asymmetric OADMs, where the signal is added or dropped only, or the number of add and drop signals is not equal, or systems where the wavelength of the add and drop channels are different.
The above prior art methods fall short in providing a reliable solution for monitoring the performance as well as equalizing the channels of WDM or DWDM systems equipped with OADMS.
An object of the present invention is to provide a method for performance. monitoring and equalization of networks with OADMs that alleviates totally or in part the drawbacks of current methods.
Another object of the invention is to provide a performance monitoring and equalization method that may be used for networks with symmetric or asymmetric OADMs and networks where the wavelength of the drop and add channels are equal or not.
A further object of the invention is to provide methods for performance monitoring and equalization of OADM networks that treat a complex WDM network as a xe2x80x9ctwo-endxe2x80x9d system. This allows for simplifying the monitoring of the network, as the performance information for all physical sites of the network is available at these two ends.
Accordingly there is provided a method for performance monitoring and equalization of an optical link between two line terminating equipment (LTE/REGEN) sites of an optical network provided with one or more sites equipped with optical add/drop multiplexers (OADMs) and multi-channel optical amplifiers (MOA), comprising: selecting a direction of transmission and accordingly defining one of the LTE/REGEN sites as a source analogous end (AE) and the other as a destination AE; converting the optical link into an analogous system of J analogous channels (AC), an AC(j) originating at the source AE and terminating at said destination AE; and, adjusting the input power of each AC(j) to obtain a substantially equal performance parameter for all ACs, where J greater than 2,j is the identifier of an AC, and jxcex5[1,j].
An advantage of this method is that it provides a reliable equalization method for a WDM/DWDM network equipped with OADMs that could be used in the field.
Another advantage of the invention is that it provides a simple solution for performance monitoring from the network management point of view. Thus, OAMandP (operations, administration, maintenance and provisioning) information for an entire network comprised between two sites of interest is available at these two sites/ends.
Still another advantage of the method according to the present invention is that it can used in for equalization of networks provided with symmetric or asymmetric OADMs and also in cases when the add and drop channels have different wavelengths.